Recovery of Olefins from Para-Xylene Process

ABSTRACT

A process for producing para-xylene, by (a) contacting toluene with methanol in the presence of an alkylation catalyst under conditions effective to produce an alkylation effluent comprising xylenes and a by-product mixture comprising water, dimethyl ether and C 4 − hydrocarbons; (b) separating the alkylation effluent into a first fraction containing xylenes and a second fraction containing the by-product mixture; (c) removing water from the second fraction to produce a dried by-product mixture; (d) fractionating the dried by-product mixture to separate the mixture into a bottoms stream containing dimethyl ether and an overhead stream containing at least some of the C 4 - hydrocarbons; and (e) recovering ethylene and propylene from the overhead stream.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and benefit of U.S. application No.61/711,346 filed Oct. 9, 2012, the disclosure of which is fullyincorporated herein by reference.

FIELD

This invention relates to a process for producing para-xylene by thealkylation of benzene and/or toluene with methanol and recovery ofolefins from the process.

BACKGROUND

Of the xylene isomers, para-xylene is of particular value since it isuseful in the manufacture of terephthalic acid which is an intermediatein the manufacture of synthetic fibers. Para-xylene is a valuablesubstituted aromatic compound which is in great demand for theproduction of terephthalic acid, a major component in forming polyesterfibers and resins. Today, para-xylene is commercially produced byhydrotreating of naphtha (catalytic reforming), steam cracking ofnaphtha or gas oil, and toluene disproportionation.

One problem with most existing processes for producing xylenes is thatthey produce a thermodynamic equilibrium mixture of ortho (o)-, meta(m)- and para (p)-xylenes, in which the para-xylene concentration istypically only about 24 wt %. Thus, separation of para-xylene from suchmixtures tends to require superfractionation and multistagerefrigeration steps. Such processes involve high operation costs andresult in only limited yields. There is therefore a continuing need toprovide processes which are highly selective for the production ofp-xylene.

One known method for producing xylenes involves the alkylation ofbenzene and/or toluene with methanol over a solid acid catalyst. Thusthe alkylation of toluene with methanol over cation-exchanged zeolite Yhas been described by Yashima et al. in the Journal of Catalysis 16,273-280 (1970). These workers reported selective production ofpara-xylene over the approximate temperature range of 200 to 275° C.,with the maximum yield of para-xylene in the mixture of xylenes, i.e.,about 50% of the xylene product mixture, being observed at 225° C.Higher temperatures were reported to result in an increase in the yieldof meta-xylene and a decrease in production of para and ortho-xylenes.

Alternatively, ZSM-5-type zeolite, zeolite beta andsilicaaluminophosphate (SAPO) catalysts have been used for this process.For example, U.S. Pat. No. 6,504,072 discloses a toluene methylationprocess that is highly selective for the production of para-xylene andwhich comprises reacting toluene with methanol in the presence of acatalyst comprising a porous crystalline material having a DiffusionParameter for 2,2 dimethylbutane of about 0.1-15 sec⁻¹ when measured ata temperature of 120° C. and a 2,2 dimethylbutane pressure of 60 torr (8kPa). The porous crystalline material is preferably a medium-porezeolite, particularly ZSM-5, which has been severely steamed at atemperature of at least 950° C. The porous crystalline material ispreferably combined with at least one oxide modifier, preferablyincluding phosphorus, to control reduction of the micropore volume ofthe material during the steaming step.

Although toluene methylation, and particularly the para-selectivetoluene methylation process of U.S. Pat. No. 6,504,072, provides anattractive route to para-xylene, the process inevitably producessignificant quantities of light (C₄−) gas. These gaseous by-productsinclude olefins, particularly ethylene, propylene and butylenes;alkanes, such as methane, ethane, propane and butanes; and oxygenates,such as dimethyl ether. To improve overall process economics, there is aneed for an efficient method of recovering at least some of theseby-products so that the value of the light gas stream can be increasedabove fuel value. The present invention seeks to provide such a process.

According to the present invention, it has now been found thatsignificant quantities of light olefins, particularly ethylene andpropylene, can be recovered from methanol/toluene alkylation processesand diverted to uses other than merely as fuel.

SUMMARY

The invention resides in a process for producing para-xylene, theprocess comprising (a) contacting benzene and/or toluene with methanolin the presence of an alkylation catalyst under conditions effective toproduce an alkylation effluent comprising xylenes and a by-productmixture comprising water, dimethyl ether and C₄− hydrocarbons; (b)separating the alkylation effluent into a first fraction containingxylenes and a second fraction containing the by-product mixture; (c)removing water from the second fraction to produce a dried by-productmixture; (d) fractionating the dried by-product mixture to separate themixture into a bottoms stream containing dimethyl ether and an overheadstream containing at least some of the C₄− hydrocarbons; and (e)recovering ethylene and propylene from the overhead stream.

Advantageously, the process can be conducted such that water is removedfrom said second fraction by passing the second fraction through amolecular sieve drier, or in the alternative, such that water is removedfrom said second fraction by washing the second fraction with methanol.

In a preferred embodiment, the process includes passing the methanolthrough a molecular sieve drier prior to washing the second fractionwith the methanol and can be conducted such that the dried by-productmixture comprises less than 100 ppm by weight of water, more preferably20 ppm by weight, or less, of water, and most preferably 1 ppm byweight, or less, of water, and the overhead stream comprises less than100 ppm by weight of dimethyl ether, more preferably 20 ppm by weight,or less, of dimethyl ether, and most preferably 1 ppm by weight, orless, of dimethyl ether.

In another embodiment, the by-product mixture produced in step (a) alsocomprises carbon monoxide, and the process further comprises removingthe carbon monoxide from the by-product mixture prior to the waterremoval step (c), or removing carbon monoxide from the overhead stream,which can be in the vapor phase, prior to the recovery step (e).

In another embodiment, the process is conducted such that the overheadstream is a vapor-phase stream, and can further comprise (d)(i) passingthe overhead stream of step (d) in a vapor phase into a partialcondenser and cooling the stream to remove remaining condensables fromthe vapor phase; and (d)(ii) recovering the vapor phase, andadvantageously (d)(iii) passing the vapor phase from step (d)(ii) to acryogenic separation unit to separate ethylene and propylene from anyremaining overhead stream components. In an embodiment, ethylene andpropylene can be recovered separately or together, such ascryogenically, to be used, for instance in further processingsteps(e.g., polymerization) or for sale.

Conveniently, the cryogenic separation unit is one of a refinery gasrecovery unit or a fluidized catalytic cracking unit recovery unit.

In a particularly preferred embodiment, step (a) of the process includesproviding a feedstream containing at least about 90 wt % toluene, andcan be conducted over an alkylation catalyst which is a porouscrystalline material having a Diffusion Parameter for 2,2 dimethylbutaneof about 0.1-15 sec⁻¹ when measured at a temperature of 120° C. and a2,2 dimethylbutane pressure of 60 torr (8 kPa), such as a medium-poresize aluminosilicate zeolite selected from the group consisting ofZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35, and ZSM-48, optionallycomposited with an inorganic oxide matrix.

In another embodiment, the process can be conducted such that a methanolfeed is injected in stages into the alkylation catalyst at one or morelocations downstream from the location of injection of the toluene.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of a process for recovery of light olefins froma methanol/toluene alkylation process, according to one example of thepresent application.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Described herein is a process for producing para-xylene by the catalyticalkylation of benzene and/or toluene with methanol. The alkylationprocess produces a para-rich mixture of xylene isomers, together withwater and some light organic by-products, particularly dimethyl etherand C₄− olefinic hydrocarbons. The present process provides an improvedmethod of separating and recovering at least the olefins from theselight by-products for uses other than as fuel.

Alkylation Process

The alkylation process employed herein can employ any aromatic feedstockcomprising benzene and/or toluene, although in general it is preferredthat the aromatic feed contains at least 90 wt %, especially at least 99wt %, of toluene. Similarly, although the composition of themethanol-containing feed is not critical, it is generally desirable toemploy feeds containing at least 90 wt %, especially at least 99 wt %,of methanol.

The catalyst employed in the alkylation process is generally a porouscrystalline material and, in one preferred embodiment, is a porouscrystalline material having a Diffusion Parameter for 2,2 dimethylbutaneof about 0.1-15 sec⁻¹ when measured at a temperature of 120° C. and a2,2 dimethylbutane pressure of 60 torr (8 kPa).

As used herein, the Diffusion Parameter of a particular porouscrystalline material is defined as D/r²×10⁶, wherein D is the diffusioncoefficient (cm²/sec) and r is the crystal radius (cm). The diffusionparameter can be derived from sorption measurements provided theassumption is made that the plane sheet model describes the diffusionprocess. Thus for a given sorbate loading Q, the value Q/Q_(eq), whereQ_(eq) is the equilibrium sorbate loading, is mathematically related to(Dt/r²)^(1/2) where t is the time (sec) required to reach the sorbateloading Q. Graphical solutions for the plane sheet model are given by J.Crank in “The Mathematics of Diffusion”, Oxford University Press, ElyHouse, London, 1967.

The porous crystalline material is preferably a medium-pore sizealuminosilicate zeolite. Medium pore zeolites are generally defined asthose having a pore size of about 5 to about 7 Angstroms, such that thezeolite freely sorbs molecules such as n-hexane, 3-methylpentane,benzene and p-xylene. Another common definition for medium pore zeolitesinvolves the Constraint Index test which is described in U.S. Pat. No.4,016,218, which is incorporated herein by reference. In this case,medium pore zeolites have a Constraint Index of about 1-12, as measuredon the zeolite alone without the introduction of oxide modifiers andprior to any steaming to adjust the diffusivity of the catalyst. Inaddition to the medium-pore size aluminosilicate zeolites, other mediumpore acidic metallosilicates, such as silicoaluminophosphates (SAPOs),can be used in the present process.

Particular examples of suitable medium pore zeolites include ZSM-5,ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35, and ZSM-48, with ZSM-5 andZSM-11 being particularly preferred. In one embodiment, the zeoliteemployed in the process of the invention is ZSM-5 having a silica toalumina molar ratio of at least 250, as measured prior to any treatmentof the zeolite to adjust its diffusivity.

Zeolite ZSM-5 and the conventional preparation thereof are described inU.S. Pat. No. 3,702,886. Zeolite ZSM-11 and the conventional preparationthereof are described in U.S. Pat. No. 3,709,979. Zeolite ZSM-12 and theconventional preparation thereof are described in U.S. Pat. No.3,832,449. Zeolite ZSM-23 and the conventional preparation thereof aredescribed U.S. Pat. No. 4,076,842. Zeolite ZSM-35 and the conventionalpreparation thereof are described in U.S. Pat. No. 4,016,245. ZSM-48 andthe conventional preparation thereof is taught by U.S. Pat. No.4,375,573. The entire disclosures of these U.S. patents are incorporatedherein by reference.

The medium pore zeolites described above are preferred for the presentprocess since the size and shape of their pores favor the production ofp-xylene over the other xylene isomers. However, conventional forms ofthese zeolites have Diffusion Parameter values in excess of the 0.1-15sec⁻¹ range desired for the present process. Nevertheless, the requireddiffusivity can be achieved by severely steaming the zeolite so as toeffect a controlled reduction in the micropore volume of the catalyst tonot less than 50%, and preferably 50-90%, of that of the unsteamedcatalyst. Reduction in micropore volume is monitored by measuring then-hexane adsorption capacity of the zeolite, before and after steaming,at 90° C. and 75 torr n-hexane pressure.

Steaming to achieve the desired reduction in the micropore volume of theporous crystalline material can be effected by heating the material inthe presence of steam at a temperature of at least about 950° C.,preferably about 950 to about 1075° C., and most preferably about 1000to about 1050° C. for about 10 minutes to about 10 hours, preferablyfrom 30 minutes to 5 hours.

To effect the desired controlled reduction in diffusivity and microporevolume, it may be desirable to combine the porous crystalline material,prior to steaming, with at least one oxide modifier, preferably selectedfrom oxides of the elements of Groups IIA, IIIA, IIIB, IVA, VA, VB andVIA of the Periodic Table (IUPAC version). Conveniently, said at leastone oxide modifier is selected from oxides of boron, magnesium, calcium,lanthanum and preferably phosphorus. In some cases, it may be desirableto combine the porous crystalline material with more than one oxidemodifier, for example a combination of phosphorus with calcium and/ormagnesium, since in this way it may be possible to reduce the steamingseverity needed to achieve a target diffusivity value. The total amountof oxide modifier present in the catalyst, as measured on an elementalbasis, may be between about 0.05 and about 20 wt %, such as betweenabout 0.1 and about 10 wt %, based on the weight of the final catalyst.

Where the modifier includes phosphorus, incorporation of modifier in thealkylation catalyst is conveniently achieved by the methods described inU.S. Pat. Nos. 4,356,338; 5,110,776; 5,231,064 and 5,348,643, the entiredisclosures of which are incorporated herein by reference. Treatmentwith phosphorus-containing compounds can readily be accomplished bycontacting the porous crystalline material, either alone or incombination with a binder or matrix material, with a solution of anappropriate phosphorus compound, followed by drying and calcining toconvert the phosphorus to its oxide form. Contact with thephosphorus-containing compound is generally conducted at a temperatureof about 25° C. and about 125° C. for a time between about 15 minutesand about 20 hours. The concentration of the phosphorus in the contactmixture may be between about 0.01 and about 30 wt %.

Representative phosphorus-containing compounds which may be used toincorporate a phosphorus oxide modifier into the catalyst of theinvention include derivatives of groups represented by PX₃, RPX₂, R₂PX,R₃P, X₃PO, (XO)₃PO, (XO)₃P, R₃P═O, R₃P═S, RPO₂, RPS₂, RP(O)(OX)₂,RP(S)(SX)₂, R₂P(O)OX, R₂P(S)SX, RP(OX)₂, RP(SX)₂, ROP(OX)₂, RSP(SX)₂,(RS)₂PSP(SR)₂, and (RO)₂POP(OR)₂, where R is an alkyl or aryl, such asphenyl radical, and X is hydrogen, R, or halide. These compounds includeprimary, RPH₂, secondary, R₂PH, and tertiary, R₃P, phosphines such asbutyl phosphine, the tertiary phosphine oxides, R₃PO, such as tributylphosphine oxide, the tertiary phosphine sulfides, R₃PS, the primary,RP(O)(OX)₂, and secondary, R₂P(O)OX, phosphonic acids such as benzenephosphonic acid, the corresponding sulfur derivatives such as RP(S)(SX)₂and R₂P(S)SX, the esters of the phosphonic acids such as dialkylphosphonate, (RO)₂P(O)H, dialkyl alkyl phosphonates, (RO)₂P(O)R, andalkyl dialkylphosphinates, (RO)P(O)R₂; phosphinous acids, R₂POX, such asdiethylphosphinous acid, primary, (RO)P(OX)₂, secondary, (RO)₂POX, andtertiary, (RO)₃P, phosphites, and esters thereof such as the monopropylester, alkyl dialkylphosphinites, (RO)PR₂, and dialkyl alkyphosphinite,(RO)₂PR, esters. Corresponding sulfur derivatives may also be employedincluding (RS)₂P(S)H, (RS)₂P(S)R, (RS)P(S)R₂, R₂PSX, (RS)P(SX)₂,(RS)₂PSX, (RS)₃P, (RS)PR₂, and (RS)₂PR. Examples of phosphite estersinclude trimethylphosphite, triethylphosphite, diisopropylphosphite,butylphosphite, and pyrophosphites such as tetraethylpyrophosphite. Thealkyl groups in the mentioned compounds preferably contain one to fourcarbon atoms.

Other suitable phosphorus-containing compounds include ammonium hydrogenphosphate, the phosphorus halides such as phosphorus trichloride,bromide, and iodide, alkyl phosphorodichloridites, (RO)PCl₂,dialkylphosphoro-chloridites, (RO)₂PCl, dialkylphosphinochloroidites,R₂PCl, alkyl alkylphosphonochloridates, (RO)(R)P(O)Cl, dialkylphosphinochloridates, R₂P(O)Cl, and RP(O)Cl₂. Applicable correspondingsulfur derivatives include (RS)PCl₂, (RS)₂PCl, (RS)(R)P(S)Cl, andR₂P(S)Cl.

Particular phosphorus-containing compounds include ammonium phosphate,ammonium dihydrogen phosphate, diammonium hydrogen phosphate, diphenylphosphine chloride, trimethylphosphite, phosphorus trichloride,phosphoric acid, phenyl phosphine oxychloride, trimethylphosphate,diphenyl phosphinous acid, diphenyl phosphinic acid,diethylchlorothiophosphate, methyl acid phosphate, and otheralcohol-P₂O₅ reaction products.

Representative boron-containing compounds which may be used toincorporate a boron oxide modifier into the catalyst of the inventioninclude boric acid, trimethylborate, boron oxide, boron sulfide, boronhydride, butylboron dimethoxide, butylboric acid, dimethylboricanhydride, hexamethylborazine, phenyl boric acid, triethylborane,diborane and triphenyl boron.

Representative magnesium-containing compounds include magnesium acetate,magnesium nitrate, magnesium benzoate, magnesium propionate, magnesium2-ethylhexoate, magnesium carbonate, magnesium formate, magnesiumoxylate, magnesium bromide, magnesium hydride, magnesium lactate,magnesium laurate, magnesium oleate, magnesium palmitate, magnesiumsalicylate, magnesium stearate and magnesium sulfide.

Representative calcium-containing compounds include calcium acetate,calcium acetylacetonate, calcium carbonate, calcium chloride, calciummethoxide, calcium naphthenate, calcium nitrate, calcium phosphate,calcium stearate and calcium sulfate.

Representative lanthanum-containing compounds include lanthanum acetate,lanthanum acetylacetonate, lanthanum carbonate, lanthanum chloride,lanthanum hydroxide, lanthanum nitrate, lanthanum phosphate andlanthanum sulfate.

The porous crystalline material employed in the process of the inventionmay be combined with a variety of binder or matrix materials resistantto the temperatures and other conditions employed in the process. Suchmaterials include active and inactive materials such as clays, silicaand/or metal oxides such as alumina. The latter may be either naturallyoccurring or in the form of gelatinous precipitates or gels includingmixtures of silica and metal oxides. Use of a material which is active,tends to change the conversion and/or selectivity of the catalyst andhence is generally not preferred. Inactive materials suitably serve asdiluents to control the amount of conversion in a given process so thatproducts can be obtained economically and orderly without employingother means for controlling the rate of reaction. These materials may beincorporated into naturally occurring clays, e.g., bentonite and kaolin,to improve the crush strength of the catalyst under commercial operatingconditions. Said materials, i.e., clays, oxides, etc., function asbinders for the catalyst. It is desirable to provide a catalyst havinggood crush strength because in commercial use it is desirable to preventthe catalyst from breaking down into powder-like materials. These clayand/or oxide binders have been employed normally only for the purpose ofimproving the crush strength of the catalyst.

Naturally occurring clays which can be composited with the porouscrystalline material include the montmorillonite and kaolin family,which families include the subbentonites, and the kaolins commonly knownas Dixie, McNamee, Georgia and Florida clays or others in which the mainmineral constituent is halloysite, kaolinite, dickite, nacrite, oranauxite. Such clays can be used in the raw state as originally mined orinitially subjected to calcination, acid treatment or chemicalmodification.

In addition to the foregoing materials, the porous crystalline materialcan be composited with a porous matrix material such as silica-alumina,silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia,silica-titania as well as ternary compositions such assilica-alumina-thoria, silica-alumina-zirconia silica-alumina-magnesiaand silica-magnesia-zirconia.

The relative proportions of porous crystalline material and inorganicoxide matrix vary widely, with the content of the former ranging fromabout 1 to about 90% by weight and more usually, particularly when thecomposite is prepared in the form of beads, in the range of about 2 toabout 80 wt % of the composite.

The alkylation process can be conducted in any known reaction vessel butgenerally the methanol and aromatic feeds are contacted with thecatalyst described above with the catalyst particles being disposed inone or more fluidized beds. Each of the methanol and aromatic feeds canbe injected into the fluidized catalyst in a single stage. However, inone embodiment, the methanol feed is injected in stages into thefluidized catalyst at one or more locations downstream from the locationof the injection of the aromatic reactant into the fluidized catalyst.For example, the aromatic feed can be injected into a lower portion of asingle vertical fluidized bed of catalyst, with the methanol beinginjected into the bed at a plurality of vertically spaced intermediateportions of the bed and the product being removed from the top of thebed. Alternatively, the catalyst can be disposed in a plurality ofvertically spaced catalyst beds, with the aromatic feed being injectedinto a lower portion of the first fluidized bed and part of the methanolbeing injected into an intermediate portion of the first bed and part ofthe methanol being injected into or between adjacent downstream catalystbeds.

The conditions employed in the alkylation stage of the present processare not narrowly constrained but, in the case of the methylation oftoluene, generally include the following ranges: (a) temperature betweenabout 500 and about 700° C., such as between about 500 and about 600°C.; (b) pressure of between about 1 atmosphere and about 1000 psig(between about 100 and about 7000 kPa), such as between about 10 psigand about 200 psig (between about 170 and about 1480 kPa); (c) molestoluene/moles methanol (in the reactor charge) of at least about 0.2,such as from about 0.2 to about 20; and (d) a weight hourly spacevelocity (“WHSV”) for total hydrocarbon feed to the reactor(s) of about0.2 to about 1000, such as about 0.5 to about 500 for the aromaticreactant, and about 0.01 to about 100 for the combined methanol reagentstage flows, based on total catalyst in the reactor(s).

Product Treatment and Recovery

The product of the reaction between the methanol and toluene and/orbenzene is an alkylation effluent comprising para-xylene and otherxylene isomers, water vapor, unreacted toluene and/or benzene, unreactedmethanol, phenolic impurities, and a variety of light gas by-products,such as C₄− hydrocarbons, including light olefins, and dimethyl ether.The alkylation effluent will also generally contain some C₉+ aromaticby-products. In addition, where the process is conducted in a fluidizedcatalyst bed, the alkylation effluent will contain some entrained solidcatalyst and catalyst fines. Thus the effluent, which is generally inthe vapor phase, leaving the (final) fluidized bed reactor is generallypassed through an integral cyclone separator to remove some of theentrained catalyst solids and return them to the alkylation reactor.

The alkylation effluent leaves the alkylation reactor system at a hightemperature, typically between about 500 and about 600° C. and initiallymay be passed through a heat exchanger so that the waste heat in theeffluent stream may be recovered and used to heat other processstream(s). It is, however, preferred that any initial cooling of theproduct stream is limited so as to keep the effluent vapors well abovethe dew point, typically about 240° F. (116° C.).

Following further cooling, the effluent vapor stream is fed to aseparation system, which may comprise one or more fractionation columns,where the unreacted methanol and aromatics are recovered and recycled tothe alkylation step, the light (C₄−) and heavy (C₉+) by-products areremoved and the remainder of effluent is separated into a liquid organicproduct stream rich in xylenes and a waste water stream. The waste wateris decanted from the organic product stream and the para-xylene isrecovered from the organic product stream, typically by fractionalcrystallization or selective adsorption.

In the present process, rather than being sent directly to fuel use, thelight (C₄−) stream is treated to recover at least the valuable olefiniccomponent of the stream. Typically, this treatment initially involvessubjecting the light stream to a drying step to remove water, such aswith a molecular sieve drier or by washing with methanol, which itselfhas preferably been dried to remove water, such as with a molecularsieve drier. The dried by-product mixture preferably contains less than100 ppm water by weight, preferably 20 ppm or less by weight, still morepreferably 1 ppm or less by weight. Optionally, some or all of thecarbon monoxide formed in the methylation process can be removed priorto the drying step. The amount of water present can be convenientlymeasured by a Panametrics moisture analyzer, gas chromatograph,simulation using VLE data, and other methods known by those of ordinaryskill in the art.

The dried by-product mixture is then sent to a fractionation towerprimarily to remove dimethyl ether from the light olefins, so as tominimize the impact of dimethyl ether on olefins recovery equipment.Dimethyl ether can also be deleterious to a later-recovered propyleneproduct by negatively impacting propylene in downstream processes suchas polymerization. The fractionation tower acts to fractionate the driedby-product mixture into an overhead stream, containing at least some ofthe C₄− hydrocarbons, and almost all of the dimethyl ether and C₄+hydrocarbons as a liquid bottoms stream. For example, ethylene and atleast about 98 wt % of the propylene, and about 67 wt % of the propanefrom the fractionation column are recovered in the overhead stream,while nearly 100 wt % of the dimethyl ether and nearly 100 wt % of C₄+hydrocarbons are removed in the liquid bottoms stream. The overheadvapor from the fractionation tower, which generally comprises less thanabout 100 ppm dimethyl ether, preferably 20 ppm or less by weight, morepreferably 1 ppm or less by weight, and if not previously removed somecarbon monoxide, is sent to a refrigerated partial condenser, such as acondenser using propylene refrigeration, and then to a separation drum,where a vapor product phase is recovered and remaining condensables areremoved and provided as reflux to the fractionation tower. Thisarrangement avoids the need for the ethylene refrigeration which wouldbe required to condense all the vapor from the tower overhead andthereby decreases cooling costs. The amount of DME in the overhead vaporis most preferably analyzed by gas chromatography which has beencalibrated to measure oxygenates such as DME.

Subsequently, the vapor product phase recovered from the separation drumis passed to a cryogenic separation unit, preferably an existingcryogenic separation unit associated with, for example, a refinery gasrecovery system or a fluidized catalytic cracking unit recovery system,to effect separation and recovery of ethylene and propylene from anyremaining gases in the overhead vapor. If not previously removed, carbonmonoxide is removed prior to sending the overhead vapor phase to thecryogenic separation unit.

The dimethyl ether and C₄+ hydrocarbons removed from the fractionationtower as the liquid bottoms stream can be used as fuel or the dimethylether can be recovered using a further fractionation tower and recycledto the methylation reactor.

The carbon monoxide recovered from the methylation reaction effluent canalso be used as fuel or can be converted to methane in a conventionalmethanation reactor according to the following reaction:

CO+3H₂→CH₄+H₂O

One example of the present process for treating the light gases from atoluene methylation unit is shown in FIG. 1, in which the light gases(1) are fed via an optional CO recovery unit (1A) to a dryer (2). Afterpassage through the dryer (2), the dried gaseous mixture is sent to afractionation tower (3), where the mixture is divided into an overheadvapor stream (4) and a liquid bottoms stream (5). The overhead vaporstream (4) is sent to a refrigerated partial condenser (6), such as acondenser using propylene refrigeration, and then to a separation drum(7), wherein a vapor phase (8) is recovered and the remainingcondensables are removed and provided as reflux (9) to the fractionationtower. The vapor phase (8) is then fed via an optional CO recovery unit(8A) to a cryogenic separation unit (10) for recovery of the ethyleneand propylene.

While the present invention has been described and illustrated byreference to particular embodiments, those of ordinary skill in the artwill appreciate that the invention lends itself to variations andmodifications not necessarily illustrated herein without departing fromthe spirit and scope of the invention.

Trade names used herein are indicated by a ™ symbol or ® symbol,indicating that the names may be protected by certain trademark rights,e.g., they may be registered trademarks in various jurisdictions. Allpatents and patent applications, test procedures (such as ASTM methods,UL methods, and the like), and other documents cited herein are fullyincorporated by reference to the extent such disclosure is notinconsistent with this invention and for all jurisdictions in which suchincorporation is permitted. When numerical lower limits and numericalupper limits are listed herein, ranges from any lower limit to any upperlimit are contemplated.

1. A process for producing para-xylene, the process comprising: (a)contacting toluene and/or benzene with methanol in the presence of analkylation catalyst under conditions effective to produce an alkylationeffluent comprising xylenes and a by-product mixture comprising water,dimethyl ether and C₄− hydrocarbons; (b) separating the alkylationeffluent into a first fraction containing xylenes and a second fractioncontaining the by-product mixture; (c) removing water from the secondfraction to produce a dried by-product mixture; (d) fractionating thedried by-product mixture to separate the mixture into a bottoms streamcontaining dimethyl ether and an overhead stream containing at leastsome of the C₄− hydrocarbons; and (e) recovering ethylene and propylenefrom the overhead stream.
 2. The process of claim 1, wherein water isremoved from said second fraction by passing the second fraction througha molecular sieve drier.
 3. The process of claim 1, wherein water isremoved from said second fraction by washing the second fraction withmethanol.
 4. The process of claim 3, further comprising passing themethanol through a molecular sieve drier prior to washing the secondfraction with the methanol.
 5. The process of claim 1, wherein the driedby-product mixture comprises less than 100 ppm by weight of water. 6.The process of claim 1, wherein the overhead stream comprises less than100 ppm by weight of dimethyl ether.
 7. The process of claim 1, whereinthe by-product mixture produced in (a) also comprises carbon monoxide.8. The process of claim 7, further comprising removing carbon monoxidefrom the by-product mixture prior to the water removal step (c).
 9. Theprocess of claim 7, further comprising removing carbon monoxide from theoverhead stream prior to the recovery step (e).
 10. The process of claim1, wherein the overhead stream is a vapor-phase stream.
 11. The processof claim 1, further comprising (d)(i) passing the overhead stream ofstep (d) in a vapor phase into a partial condenser and cooling thestream to remove remaining condensables from the vapor phase; and(d)(ii) recovering the vapor phase.
 12. The process of claim 11, furthercomprising (d)(iii) passing the vapor phase from step (d)(ii) to acryogenic separation unit to separate ethylene and propylene from anyremaining overhead stream components and optionally further including astep (d) (iv) of separating ethylene from propylene cryogenically. 13.The process of claim 12, wherein the cryogenic separation unit is one ofa refinery gas recovery system or a fluidized catalytic cracking unitrecovery system, or an ethylene plant recovery system, or a pyrolysiscracking furnace system, or any combination thereof.
 14. The process ofclaim 12, further comprising removing carbon monoxide from the vaporphase.
 15. The process of claim 13, further comprising removing carbonmonoxide from the vapor phase.
 16. The process of claim 1, wherein thetoluene is provided in a feedstream containing at least about 90 wt %toluene.
 17. The process of claim 1, wherein the alkylation catalyst isa porous crystalline material having a Diffusion Parameter for 2,2dimethylbutane of about 0.1-15 sec⁻¹ when measured at a temperature of120° C. and a 2,2 dimethylbutane pressure of 60 torr (8 kPa).
 18. Theprocess of claim 17, wherein the alkylation catalyst is a medium-poresize aluminosilicate zeolite selected from the group consisting ofZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35, and ZSM-48, optionallycomposited with an inorganic oxide matrix.
 19. The process of claim 1,wherein a methanol feed is injected in stages into the alkylationcatalyst at one or more locations downstream from the location ofinjection of the toluene.